[iOS] Replace startup histograms that are broken by prewarming

iOS pre-warming breaks histograms that try to measure time since
process creation, since process creation can happen because of
pre-warming rather than because the user wants to use the app. So
process creation can be hours (or more) before startup finishes.

This CL updates histograms to measure time since main() is called
instead, and deletes histograms that were only measuring time before
main().

OBSOLETE_HISTOGRAM[Startup.ColdStartFromProcessCreationTimeV2]=Replaced by Startup.ColdStartFromMain
OBSOLETE_HISTOGRAM[Startup.TimeFromProcessCreationToDidFinishLaunchingCall]=Replaced by Startup.TimeFromMainToDidFinishLaunchingCall
OBSOLETE_HISTOGRAM[Startup.TimeFromProcessCreationToLoad]=Removed since it is broken by pre-warming
OBSOLETE_HISTOGRAM[Startup.TimeFromProcessCreationToMainCall]=Removed since it is broken by pre-warming
OBSOLETE_HISTOGRAM[Startup.TimeFromProcessCreationToSceneConnection]=Replaced by Startup.TimeFromMainToSceneConnection

Bug: 324413102
Change-Id: Ifab334e7dac9c3c769999cfe3eb2b1a85fd0d996
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5397338


Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281918}